This destructive tree disease is caused by the basidiomycete fungus, Chondrostereum purpureum. It is common on cotoneaster hedges in Saskatchewan but silver leaf also occurs on many other tree species including fruit trees like plum and apple; and ornamentals like birch, choke cherry, crabapple, hawthorn, poplar, Manitoba maple (Boxelder), mountain ash, Nanking cherry, spruce and willow.
At the edges of infected branches, foliage develops a dull leaden or metallic lustre which is best described as silver leaf. Later brown areas occur on leaves. If the disease is not noticed at that time, it will become more apparent as branches and even entire stems die.
The disease is spread by airborne fungal spores with infection only occurring through wounds. Upper branch symptoms, however, are not from the fungal growth but appear to be due to fungus-induced toxins which move upward from the infected area and result in the silver leaf symptoms at the branch terminals. The fungus advances through the stem and it will kill the invaded area and if left unheeded the entire tree will die. After death, the fungus develops spore producing structures (sporophores) on the bark of the root collar or at the base of dead shoots. The sporophores are flat and somewhat shelf-shaped brackets, each about 0.5 to 2 centimetres (1/4 to 3/4 inches). The velvety upper surface of the bracket is tawny-grey while the smooth lower surface is purple when young but later turns brown.
Since the fungus can only enter through wounds, it is apparent that the many wounds created when pruning make cotoneaster and other hedges very susceptible to silver leaf. Therefore, the prompt removal of diseased material is essential so sporulation cannot occur and spread the disease.
- Prune at the first sign of silvering. Make the cut below the diseased area into healthy wood. This usually means cutting well below the terminal where silver leaf symptoms occur since the infected or cankered area can be further down the branch. It could mean cutting off a whole shoot at the base, but if the disease has already spread to the base and other shoots show symptoms, the whole root should be removed. As the fungus can sporulate on infected wood, pruning should be removed, burned or buried. If the disease necessitates the removal of a plant in an established hedge, a new plant can be put in its place. However, the neighbouring plants on either side should be cut back nearly to the root collar and kept pruned short for at least a year so the new plant can get established.
- Protect trees from unnecessary injury since the fungus penetrates through wounds.
- Large wounds including pruning wounds greater than 1.5 centimetres (1/2") in diameter should be covered with a wound dressing such as Braco, Shellac or other reliable wound dressing products available from seed and garden supply dealers.
- Remove and burn or bury all infected dead trees, infected branches and parts where brackets have developed. This will prevent the spread of the disease.
- No chemical control.